GLOBAL 1°X1° THERMAL MODEL FOR THE CONTINENTAL LITHOSPHERE: IMPLICATIONS FOR LITHOSPHERE GROWTH SINCE ARCHEAN

The study reports a new global thermal model for the continental lithosphere. Geotherms for continental terranes of different ages (>3.6 Ga to present), constrained by reliable data on borehole heat flow measurements (Artemieva and Mooney, 2001), are statistically analyzed as a function of age and are used as a proxy for lithospheric temperatures in continental regions with no or low-quality heat flow data (>50 per cent of the continents). These data are supplemented by cratonic geotherms based on xenolith and electromagnetic data. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1 deg x1 deg grid combined with age relationship of continental geotherms formed the basis for a new global thermal model of the continental lithosphere.

Statistical analysis of continental geotherms reveals that thick (>250 km) lithosphere is restricted solely to young Archean terranes (2.6-3.0 Ga), while in old Archean cratons (3.0-3.6 Ga) lithospheric roots do not extend deeper than 200-220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei. It is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the present-day lithospheric volume for terranes of different ages, which does not reveal a peak in lithospheric volume at 2.6-2.7 Ga as expected from growth curves for juvenile crust. The absence of the peak can be attributed to lithosphere recycling back into the mantle; recycling rates for lithospheric mantle are presented for different geological ages since Archean until present.